1. Field of the Invention
This invention relates to an absorbent and a method for the production of an absorbent resin. More particularly, it relates to an absorbent having an extremely low residual monomer content and a method for producing an absorbent resin having a notably low residual monomer content without adversely affecting absorption properties thereto.
2. Description of the Prior Art
In recent years, the use of a various absorbent resins as one component material for sanitary napkin, disposable diaper, and other sanitary products serving to absorb bodily fluids or as an agronomic water-retaining agent has been prevailing.
The absorbent resins of this class heretofore known to the art include partially neutralized cross-linked polyacrylic acid, hydrolyzate of starch-acrylonitrile graft polymer, partially neutralized starch-acrylic acid graft polymer, saponified vinyl acetate-acrylic ester copolymers, and hydrolyzates of acrylonitrile copolymers or acrylamide copolymers or cross-linked materials thereof.
However, these absorbent resins generally suffer persistence of unaltered monomers therein. When these absorbent resins are used in the fields of sanitary products, foodstuffs, etc., it is particularly desirable to lower the residual monomer contents to the fullest possible extent.
Attempts to lower the residual monomers in the polymers have been made for a long time in the field of water-soluble polymers such as, for example, polymeric flocculants. The methods adopted for these attempts are broadly sorted under the following three types as described in JP-A-56-103,207:
(1) A method which relies on increasing the amount of a polymerization initiator or making split addition of the polymerization initiator. PA1 (2) A method which resorts to extraction as with a solvent. PA1 (3) A method which is aimed at changing residual monomers into derivatives other than polymers. These techniques may be applied to the operation of lowering residual monomer contents in the absorbent resins.
Examples of application of the method of (1) to absorbent resins are found in JP-A-56-72,005 AND JP-A-1-24,808. When this method is applied to an absorbent resin, the polymer or absorbent resin shows a decrease in molecular weight of backbone polymer chain, a decrease in absorption capacity due to excessive self-cross-linking reaction, and an increase in a water-soluble content. Thus, the characteristics of the absorbent resin are degraded in spite of a decrease in the residual monomer content.
One example of the method of (2) is found in JP-A-1-292,003. Though this method is indeed effective in lowering the residual monomer content, it can be hardly called a desirable means because it requires use of a large amount of a solvent and entails very costly recovery of the solvent from the polymer.
As typical examples of the method of (3) changing a residual monomer into derivatives other than polymers, a method which resorts to addition of an amine, ammonia, or a similar in the case of a water-soluble polymer (JP-B-33,2,646) and a method which resorts to addition of a salt of a hydrogen sulfite, a sulfite, a pyrosulfite, or a similar (JP-A-56-103,207), for example, are disclosed. Examples of direct application of this method to absorbent resins are also found in JP-A-1-62,317 and US-A-4,929,717.
Our review of these methods reveals that the polymer obtained by the method disclosed in JP-A-1-62,317 still has a high residual monomer content and the same remark holds good for the method of US-A-4,929,717, that this method is characterized by mixing the polymer with a substance capable of reacting with the residual monomer, adjusting the resultant mixture to a water content in the range of 20 to 90%, and then drying the mixture at a heightened temperature, and that the residual monomer content decreased by this method is still on the level of some tens of ppm, evincing inferiority of the effect of this method. It is preferable to lower the residual monomer eontent of the absorbent resin to the fullest possible extent as described above. The absorbent resins produced by the existing methods suffer persistence of some tens to some hundreds of ppm of residual monomer therein. None of the existing absorbents has attained a decrease of the residual monomer content to below 10 ppm, let alone 5 ppm.
It is, therefore, an object of this invention to provide a novel absorbent and a method for the production of an absorbent resin.
Another object of this invention is to provide an absorbent having a residual monomer at an extremely low concentration and a method for the production of the absorbent resin.